[FEAT][DOCS] Merge all images in repo (docs) to one folder

[shir22]

Is your feature request related to a problem? Please describe.
Currently some resources appear in a double manner (in both places), thus when updating one the other should be manually updated.
In addition, resources aren't organized (are all together in a folder).
Two paths where the images appear:

1. deepchecks/docs/images
2. deepchecks/docs/source/_static

Describe the solution you'd like
Merge them into one place: source/_static/*, and organize within that place in subfolders by type (verify that new version is used if there are inconsistencies), fix references in docs, readme(s) and checks-demo.

Describe alternatives you've considered
Manually keep updating double pictures